The present invention concerns sheet guidance arrangements used in the sheet-outfeed units of printing machines. Such sheet guidance arrangements serve to improve the quality of the transport of printed sheets from the printing mechanisms of the printing machine per se through the sheet-outfeed unit of the machine, so that the sheet can be transported through the outfeed unit and deposited onto the outfeed stack without detrimental effects upon the print quality of the printing on the sheets.
Federal Republic of Germany published patent application DT-OS 2,137,115 discloses a sheet guidance arrangement for the sheet-outfeed unit of a printing machine. The sheet-transport path within the sheet-outfeed unit includes a curved section at which is arranged a guide plate which extends, transversely to the sheet-transport direction, for the full width of the transported sheets. The guide plate is provided with nozzles through which air is sucked. The sucked-air action exerts an underpressure upon the sheet, pulling the surface of the sheet into contact with the surface of the guide plate. As a result the sheet does not flutter but instead is transported stretched taught through the sheet-outfeed unit of the printing machine, then to the suction roller at the outfeed end of the outfeed unit, and is then deposited onto the outfeed stack.
With this known arrangement, an important drawback is that the entire surface of the sheet is pulled by means of suction into surface contact with the entire surface of the guide plate. If the sheet has been run through a printing machine operating in the two-sided-printing mode, i.e., so that both faces of the sheet bear freshly applied printing during transport through the sheet-outfeed unit, the printing on the side of the sheet pulled against the guide plate as a very great tendency to smear. As a direct result of this, such known sheet guidance arrangement is really suitable only for printing machines operating in the one-sided-printing mode, i.e., wherein the face of the sheet pulled into surface contact with the guide plate does not bear any printing at all.